The way most people do it is unsafe. Sending them to Moser is a different story, but they charge accordingly. For around $300 you can just buy brand new axles from Dr.Diff that will be much stronger than the stock ones ever were, and lose the narrow, hard to find parts for SBP brakes at the same time.
my concern is I plan to narrow the rear and trying to decide the width of the axle with the wheel and tire I plan to use and tubing and on and on and on
Pick your wheels first, then decide on axle width. Honestly, pick your tires first. That will determine whether you need a mini-tub or not and the wheel width you need. Then you can pick the wheels, and you can do the math on the rear axle width from the wheel specs.
